In recent years, there has been known an imaging element constituted of a plurality of pixels having different photoelectric conversion characteristics with respect to an inflection point, for instance, an imaging element having a linear photoelectric conversion characteristic (hereinafter, called as a linear characteristic) on the low luminance side with respect to an inflection point, and a logarithmic photoelectric conversion characteristic (hereinafter, called as a logarithmic characteristic) on the high luminance side with respect to the inflection point. A photoelectric conversion characteristic composed of a linear characteristic and a logarithmic characteristic is called as a linear-logarithmic characteristic.
In an imaging device provided with an imaging element having the above arrangement, a pixel value obtained from a logarithmic characteristic is converted into a pixel value obtained from a linear characteristic.
However, since there is a range having a transient photoelectric conversion characteristic composed of a linear characteristic and a logarithmic characteristic in a boundary between the linear characteristic and the logarithmic characteristic, if the entire range corresponding to a logarithmic characteristic is converted into a linear characteristic, a pixel value obtained from the transient photoelectric conversion characteristic may not be precisely converted into a pixel value obtained from the linear characteristic.
In view of the above, patent literature 1 discloses an imaging device for converting a pixel value obtained from a linear characteristic and a logarithmic characteristic, and a pixel value obtained from a transient photoelectric conversion characteristic into a pixel value obtained from a reference photoelectric conversion characteristic.
In an imaging element, there is known that a logarithmic characteristic greatly varies in each of the pixels. FIG. 13 is a graph showing linear-logarithmic characteristics, wherein the axis of ordinate designates a pixel value, and the axis of abscissas designates a luminance in terms of log. FIG. 14 is a graph showing linear characteristics obtained by converting the logarithmic characteristics shown in FIG. 13 into the linear characteristics, wherein the axis of ordinate designates a pixel value, and the axis of abscissas designates a luminance in terms of log.
As shown in FIG. 13, plotting the linear-logarithmic characteristic of each of the pixels on a graph shows that variation is small in the linear characteristics. On the other hand, observing the logarithmic characteristics, it is clear that the logarithmic characteristics of the respective pixels indicated by the solid lines do not coincide with a predicted logarithmic characteristic common to all the pixels, as indicated by the dotted line, and greatly vary.
However, the imaging device disclosed in patent literature 1 is configured to convert a pixel value obtained from a logarithmic characteristic into a pixel value obtained from a linear characteristic by using an approximation equation expressing a logarithmic characteristic common to all the pixels, without considering variation in logarithmic characteristic with respect to each of the pixels.
As a result, as shown in FIG. 14, a linear characteristic of each of the pixels after the conversion does not coincide with a linear characteristic to be converted, which makes it impossible to convert a pixel value obtained from a logarithmic characteristic into a pixel value obtained from a linear characteristic without variation. As a result, fixed pattern noise may be generated in an image.
An object of the invention is to provide an imaging device that enables to convert a pixel value obtained from a non-reference photoelectric conversion characteristic into a pixel value obtained from a reference photoelectric conversion characteristic without variation, and enables to prevent generation of fixed pattern noise in an image.